Related Articles

A young ash tree (Fraxinus pennsylvanica) should grow from 6 to 10 feet a year in its growing range of U.S. Department of Agriculture plant hardiness zones 3A through 9A. You will want to fertilize green ash nursery saplings when you transplant them and fertilize young trees, but not mature trees unless they exhibit symptoms of stress or need for plant nutrients.

Fertilizing at Planting

When you transplant a nursery sapling, its roots will need phosphorus to grow properly. Phosphorus works slowly into soil. The National Arborist Association recommends working 10 pounds of 0-46-0, 0-20-20, 5-10-5 or 4-12-4 fertilizer for each cubic yard of planting soil. Fertilizer numbers show the ratio by weight of nitrogen, phosphorus and potassium.

Fertilizing Young Trees

Young green ash trees are often fertilized to spur their growth. Horticultural research indicates that fertilizing practices in the fall are related to higher levels of rotting buds and dying shoots in the spring. Green ash trees fertilized with controlled-release fertilizer had higher levels of bud rot in the spring compared to trees fertilized with urea formaldehyde (39-0-0). Green ash trees fertilized with urea formaldehyde were also more cold tolerant than those fertilized with controlled-release fertilizer.

A300 mobile placeholder

Fertilizing Mature Trees

Green ash trees grow from 60 to 70 feet high and 45 to 50 feet wide with an upright, oval crown. Mature trees growing naturally typically require little or no fertilizer, but trees growing in urban areas may be surrounded by pavement, compacted soil or buildings that restrict their root growth. Fertilize a mature green ash tree only if it is stressed or if the roots are damaged. Trees that may need fertilizer grow slowly. They may have small, off-color leaves, dead branch tips and problems with insects and disease. Their roots may be constricted by nearby construction or suffer from sewer pipe or natural gas leaks. For best results, use a 3-1-2 or 3-1-1 fertilizer. Remember, if your tree is growing in heavy soil or soil on top of hardpan, fertilizer can collect in toxic amounts.

When to Fertilize

Young green ash trees fertilized in June grow more rapidly for their first two years compared to when not fertilized in June. Studies showed no difference in fertilizing or not fertilizing in early spring or late fall. The best time to fertilize a mature green ash is October through March so the fertilizer will be in place when the tree resumes growing in the spring. If you fertilize in late summer, you will promote late growth that is not good before winter.

Hot to Apply the Fertilizer

If you conclude that fertilizer is needed to maintain the health of your green ash tree, you can apply it by calculating the estimated square feet of its root spread or according to its diameter.The formula for determining square feet of root spread is to multiply the radius of the tree times the radius times 3.14. The radius of a tree is the distance from its trunk to the tip of its most distant branches. A tree with branches that spread 40 feet would have square-foot root spread of 20 times 20 times 3.14, or 1,256 square feet. Apply 3 pounds of nitrogen per 1,000 square feet of soil under the crown. Do this annually or apply 6 pounds every other year. Alternatively, measure the diameter of the trunk at 4 1/2 feet above the ground and apply 1/2 pound of nitrogen per inch of diameter at breast high for trees less than 6 inches in diameter. Apply 1/2 pound of nitrogen per inch of diameter at breast high for trees larger than 6 inches in diameter. The amount of nitrogen needed will be roughly the same by using either method of calculation.